1. Field of the Invention
The present invention relates to an uninterruptible power supply, which is operating in a current control mode when the backup power is activated to supply power, so as to enhance the efficiency and increase the lifetime of the battery.
2. Description of Related Art
FIG. 4 shows a conventional uninterruptible power supply, which includes a voltage boost unit 80, an average current mode controller 81 (ACMA), a backup power supply unit 82 and a detection/control unit 83. The voltage boost unit 80 is arranged between power input ends L and N and the inverter (not shown), and is composed of a voltage-boost coil L, a bridge rectifier BD, two diodes D1 and D2, and two capacitors C1 and C2, etc, wherein the bridge rectifier is controlled by an electronic switch Q to be turned on or turned off.
The average current mode controller 81 is provided to detect and control the output voltage of the voltage boost unit 80. The average current mode controller 81 detects the current value of the input choke L from a current transformer CT of the voltage boost unit 80, so as to control the power supply operation of the average current mode controller 81 via the electronic switch Q.
The backup power supply unit 82 includes a DC/DC converter 821, a battery set 822 connected to the input of the DC/DC converter 821, and a voltage mode controller (VMC) 823 for controlling the DC/DC converter 821. The DC/DC converter 821 is provided to boost the output voltage of the battery set 822. The output ends of the DC/DC converter 821 are connected to the output ends +BUS and -BUS of the voltage boost unit 80 in parallel for supplying backup power. The voltage mode controller 823 is provided to detect the output voltage of the DC/DC converter 821, so as to provide a feedback signal to the DC/DC converter 821 thereby controlling the output voltage.
The detection/control unit 83 is typically composed of a microprocessor or analog and digital integrated circuits, and is provided to monitor various signals, such as input and output voltages, and sub-control units, such as the controller in the DC/DC converter, of the uninterruptible power supply.
In the above circuit, when utility power is supplied from the power input ends normally, the average current mode controller 81 detects the current value of the input choke via the current transformer CT, and further drives the voltage boost unit 80 via the electronic switch Q to transform the input power for being supplied to the load.
When the utility power supplied from the power input ends is interrupted, the detection/control unit 83 will detect the interruption, and simultaneously activates the voltage mode controller 823 to control the DC/DC converter 821, so as to convert the voltage of the battery set 822 into a higher voltage for being supplied to the load.
With such a conventional uninterruptible power supply, it is known that the average current mode controller is operated only when the utility power is normally supplied. When the utility power is interrupted, it is the voltage mode controller 823 that is used to control the DC/DC converter 821 to operate. However, the disadvantage of simply using the voltage mode controller 823 is that only the output voltage is controlled while the input current can not be controlled, resulting in a large root mean square (RMS) value of the input current, thus, having an higher loss and lower efficiency. It may be applicable to use the average current mode controller to control the DC/DC converter 821 to increase the efficiency, but unfortunately, the average current mode controller available in the market is designed for power factor correction (PFC). Thus it is suitable to operate when the utility power is normal. Such an average current mode controller is not appropriate for controlling the DC/DC converter shown in FIG.
Therefore, there is a desire to have an improved uninterruptible power supply to mitigate and/or obviate the aforementioned problems.